1. Field of the Invention
The present invention concerns a halogen free ionic liquid at high purity which is applicable to electrochemical devices.
2. Description of the Related Art
Since an ionic liquid is less volatile, it can not be purified by distillation. Accordingly, purification at high degree of the ionic liquid is one of important subjects.
An existent ionic liquid is synthesized by synthesizing a cationic onium salt by quarternization using an alkyl halide and then conducting salt-exchange thereof by using an acid (HA) or a salt (MA) as an anion.
Since the ionic liquid synthesized by the method described above is formed by way of the cationic onium salt having a halogen ion as a counter ion, it involves a problem that halogen ions remain also after conversion to an aimed ionic liquid. Particularly, a completely halogen free ionic liquid has been demanded in the application use of device materials that suffer from significant effects by the incorporation, particularly, of halogen ions.
As a method of removing the halogen ions, there is generally a method of evaporizing them as hydrogen halide by using an acid (HA), but the method involves a problem of evolving corrosive and deleterious gases. Further, as reported in “Creation of Function and Application of Ionic Liquid”, Green Chemistry Series vol. 2, published from NTS in Feb. 1, 2004, p. 31 to 32, a method of converting the halogen ions into an alkali metal salt (MX) by using an alkali metal salt (MA) of an anion and removing the same by water washing has also been used frequently, but it is difficult by the method to remove starting materials and organic halogenated products derived therefrom. As other method of removing halogens, there are reported a method of removing halogen ions as an insoluble silver halide by using silver acetate or the like in Chemical Communication of Journal of Chemical Society, section 965 in 1992, and a method of removing halogen ions by converting them into an insoluble lead halide by using a lead salt in JP-A-9-509888, but since metal salts used for the methods are expensive and wastes containing deleterious metals are formed, they are far from inexpensive and easy synthesizing methods.